Non-destructive testing (NDT) may be used to test composite materials, such as aeroplane panels, for mechanical defects. Defects can be caused by stress on materials or mechanical damage from the impact of objects on panels or defective manufacture.
The ability to test these materials in situ is very important. Regular testing is obviously required for safety, particularly in the aeroplane industry, and it is impractical to disassemble aircraft or parts of aircraft to carry out testing.
One preferred method of NDT is to utilise a probe to excite the test material or structure with radiation (generally, but not exclusively, acoustic and near acoustic frequencies are used for NDT) and detect a response. The response can be indicative of whether the test sample is faulty or not “defected”. Usually the response is compared with a response from similar excited radiation of a reference material or structure, which will usually be an undefected composite of the same type as the test material, but may also be a composite having reference defects.
Determination of whether there is a defect in the test material usually involves merely the comparison of the output waveform provided by the detection electronics with the response signal waveform of the reference material (which may in some cases, for example, be an undefected area of the same aircraft—where it is an aircraft panel which is being tested).
Acoustic or Ultrasound frequencies used to excite the materials are typically in the range of 5 to 70 kHz.
One of the most popular known NDT systems utilises the pitch/catch impulse test. A typical pitch/catch probe comprises 2 spring-loaded (or otherwise resiliently mounted) contact tips set approximately 10 mm apart which are held in contact with the test sample. These are equipped with 2 actuators, one of which generates a mechanical vibration within the above frequency range and the other of which detects the response. The drive signal is generally a short wave train, up to 6 cycles of sinusoidal impulse (or similar). The detector measures the response of the test sample at its contact point. The propagation of the disturbance from the driver to the detector is influenced by the nature of the intervening structure and in particular, by any damage or anomaly in this region. It is therefore (in theory) possible to obtain information concerning mechanical defects in a non-destructive manner. With composite materials in particular defects may also include failure of adhesion i.e. a disbond or delamination. NDT is used to detect any defect, including disbonds and delaminations and other damages of the material itself or of rivets and joints etc.
To initially calibrate the apparatus and select drive frequency, the drive frequency is chosen so as to optimise the difference in received signal between a faulty sample and an undefected sample. The frequency is usually selected to provide the maximum output from the detector electronics.
It has been found that the performance of presently available NDT systems of this type is not good. Often it is difficult to determine with accuracy whether a defect exists or not in a test sample. Further, the present systems comprise complex and expensive hardware. They also require a relatively high level of skill to be able to operate them and interpret the results.
It would be desirable to be able to provide a method and apparatus which provides a more reliable detection of defects in test samples than in the prior art, which generally improves and simplifies the analysis of the NDT data, and in which the NDT apparatus is relatively simple and inexpensive.